1. Field of the Invention
The present invention relates to spark plugs for internal combustion engines, and to a method of making such spark plugs. More particularly, the present invention relates to a spark plug having a bimetallic ground electrode, and to a method of making such a spark plug.
2. Description of the Background Art
Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plug electrodes are subject to intense heat, and to a highly corrosive environment, generated by the exploding air/fuel mixture. To improve durability and erosion resistance, spark plug electrodes must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, fuel additives, and recirculated exhaust gases within a combustion chamber.
Spark plug designs have been suggested in which a bimetallic ground electrode includes a central core material, usually including copper, and a surrounding cladding material which is different from the central core material. This central core material is more thermally conductive than the surrounding cladding, and therefore conducts heat away from the firing tip of the ground electrode better than the previous designs. A cooler ground electrode is preferable because it does not erode as quickly as a hotter electrode. As a result, a cooler electrode contributes to a longer useful spark plug life.
Illustrative examples of this type of spark plug design may be found in U.S. Pat. Nos. 4,970,426, 5,210,457, 5,395,273, 5,530,313, 5,551,902, 5,675,209 and 5,866,973.
Although this type of design for bimetallic ground electrodes helps to ensure both thermal and electrical conductivity therethrough, a problem exists with this type of design, because the material making up the central core does not usually bond well, in a welding process, to the main spark plug shell, which is normally made of a ferrous material such as steel. The outer cladding material tends to weld to the steel shell better than the central core material.
Some efforts have been rut forward to try and improve the weld between the ground electrode and the shell. U.S. Pat. No. 5,530,313 to Chiu discloses a method of welding a copper cored ground electrode to a metal spark plug shell, in which a metal sheath of the ground electrode surrounds a copper core, and in which the metal sheath penetrates deeper into the metal shell than the copper core to provide an anchor therefor. An electrode 116 which has a metal sheath 119 extending beyond a central copper core 117 is shown in FIG. 3 of the present specification. In attaching a prior art ground electrode, such as the electrode 116 shown in FIG. 3, to a flat surface of a shell base 115, the area of contact between the electrode sheath 119 and the shell base is still somewhat limited.
A need still exists in the art for an improved design for a spark plug having a bimetallic ground electrode, in which the welded connection between the ground electrode and the shell is further enhanced and improved.
The present invention provides an improved spark plug for use with internal combustion engines. In the spark plug hereof, improved bonding is obtained between a bimetallic ground electrode and a spark plug shell having a recess formed therein to receive a tip end of the ground electrode.
A spark plug according to the invention includes a metal shell, an insulator coaxially disposed within the metal shell, and a center electrode coaxially disposed in the insulator. The metal shell has a base portion, and the base portion has a lower surface with a recess formed therein. The recess in the lower surface of the metal shell is preferred to be a substantially continuous annular grove extending around the lower surface.
A bimetallic ground electrode is affixed to the lower surface of the spark plug shell base, at the recess thereof. The ground electrode has a central core formed of a first thermally conductive metal, which preferably includes copper. The ground electrode also has an external sheath surrounding the core, the sheath being made of a second metal which includes nickel.
The present invention also encompasses a method of making a spark plug, including a first step of providing a metal shell with a cylindrical base portion, in which the base portion includes a lower surface with a recess formed therein. The method also includes a step of providing a ground electrode having a central core formed from a thermally conductive material.
The method also includes a further step of placing a tip end of the ground electrode adjacent the lower surface of the shell and aligned with the recess thereof, such that part of the tip end of the ground electrode enters into the recess. Another step in the method involves welding the ground electrode to the spark plug shell.
Accordingly, it is an object of the present invention to provide a spark plug having a bimetallic ground electrode with improved adhesion between the ground electrode and the spark plug shell.
It is a further object of the present invention to provide a spark plug having a bimetallic ground electrode in which an area of contact, between the ground electrode and the spark plug shell, is increased above the area of contact therebetween in the previously known designs.
It is yet a further object of the present invention to provide a method of making the preferred spark plug.
For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts.